Chaozhong Yang created HIVE-16972:
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Summary: FetchOperator: filter out inputSplits which length is zero
Key: HIVE-16972
URL: https://issues.apache.org/jira/browse/HIVE-16972
Project: Hive
Issue Type: Improvement
Components: HiveServer2, Physical Optimizer, Query Planning
Affects Versions: 2.1.1, 2.1.0
Reporter: Chaozhong Yang
Assignee: Chaozhong Yang
Fix For: 2.1.2
* Background
We can describe the basic work flow of common HQL query as follows:
1. compile and execute
2. fetch results
In many cases, we don't need to worry about the issues fetching results from HDFS(iff there
are mapreduce jobs generated in planning step). However, the number of results files on HDFS
and data distribution will affect the final status of HQL query, especially for HiveServer2.
We have some map-only queries, e.g:
{code:sql}
select * from myTable where date > '20170201' and date <= '20170301' and id = 88;
{code}
This query will generate more than 10,000 files on HDFS and most of those files are empty.
Of course, they are very sparse. If we send TFetchResultsRequest from HiveServer2 client with
some parameters(timeout: 90s, maxRows: 1024) , FetchOperator can not fetch 1024 rows in 90
seconds and our HiveServer2 client will mark this TFetchResultsRequest as timed out failure.
Why? In fact, It's expensive to fetch results from empty file. In our HDFS cluster( 5000+
DataNodes) , reading data from an empty file will cost almost 100 ms (100ms * 1000 ==>
100s > 90s timeout). Obviously, we can filter out those empty files or splits to speed
up the process of FetchResults.
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